And surely – in a global market drawing ever closer together – it is verging on the reckless for us to seek divorce from the world’s pre-eminent trading bloc?

Margaret Thatcher with John Major in 1990 Photo: Stephen Lock for the Telegraph

On issues such as the environment, climate change, internet costs and consumer protection, the UK can best progress – or sometimes only progress – in unity with our fellow Europeans.

In an uncertain world the UK, as part of the EU, is better able to face up to the aggressive policies of hostile nations. We are safer, because the EU has brought together former enemies to face common perils. In the last thousand years of history, no previous generation has been so fortunate.

It would be sheer folly to put this all at risk.

"Suppose we left? What are the risks? They are many and real – and simply cannot be brushed aside with flippant slogans such as 'Project Fear'."

Beyond the positive advantages of membership, we have protection from many aspects of the EU that we dislike: we are not in the Eurozone – because I kept us out of it over 20 years ago; we are not part of Schengen (and thus have control of our borders); and we have opted out of “ever closer union”. We can veto any Treaty that enhances EU powers.

We are the only nation within the EU which has managed to secure these concessions. It would surely be perverse to turn our back on these advantages, and replace them with serious risks that alarm our international friends and repel the inward investments that boost our jobs and living standards.

Suppose we left? What are the risks? They are many and real – and simply cannot be brushed aside with flippant slogans such as “Project Fear”.

Consider this: as a Member State, the UK can (and does) influence European policies – often to our advantage, and sometimes simply to minimise damage to our own domestic interests. Outside, we would not be able to influence them at all. And yet, if – as a non-Member – we wish to retain access to the Single Market, we will be compelled to follow EU rules, over which we would have no influence at all. This is not only demeaning, it is a recipe for economic self-harm.

Former Prime Minister John Major Photo: Getty Images

The “leave” campaign blandly assumes that once they have undermined – if not wrecked – the power of the EU by leaving it, they can simply re-negotiate all the advantages of membership with pliant Europeans eager for our trade. This is self-deception to the point of delusion. Their argument is that the EU needs the UK market more than we need theirs, on the basis that – overall – the EU exports more to the UK than we export to them. This is, at best, disingenuous. More bluntly, it is fantasy.

UK exports to Europe are nearly 45 per cent of our total exports. On average – across the EU – the other 27 Member States send only 7% of their total exports to us. In the game of who needs who the most, the answer is clear. Our European partners will not be the demandeur in any negotiations on the Single Market – we will be.

Moreover, if we left, it is blithe optimism on a Panglossian scale for the “leave” campaign to assume that our partners – having been re-buffed and deserted in an EU diminished by our departure – will be well disposed and eager to accede to our demands.

I fear the reverse will be true. Resentment will be deep. The broken relationship is more likely to be poisonous than harmonious. The UK will have chosen to leave and, by so doing, will have gravely weakened the whole of the EU. Our partners will not wish to reward us for that – indeed, they may well be more inclined to resist our demands to discourage other nations from leaving it.

Former Prime Minister John Major Photo: Geoffrey Swaine/REX

In time, the EU will no doubt do a trade deal with us – but it will certainly not be a sweetheart deal: and negotiating it is likely to be harder and harsher than the optimists believe. And if we wish such a deal to include services (such as banking and insurance), or to prevent hidden non-tariff barriers – which we do since both are crucial to our wellbeing – it may be a long time coming.

Of course – and “leave” campaigners please note – the price of any deal with significant access to the Single Market is that we will be forced to accept free movement of people, and pay into the EU budget. Without that, as Germany’s Finance Minister has made clear, there will be no deal. These are realities that the “leave” campaign must face up to and address, so that the British people are able to reach their decision based on facts. Instead, they ignore – even obscure – the facts, to hide the weakness of their case.

“Give us our country back” is an emotional appeal that warms the heart of all those who love our country, as I do. But it is a meaningless soundbite. An illusion. A prelude to disappointment. And what country, exactly, will we “get back”? Will Scotland remain part of the UK? As a Unionist, I hope so – but no-one should ignore the threat that if the UK-wide vote is to leave, Scotland may demand another referendum on independence. The UK out of the EU and Scotland out of the UK would be a truly awful outcome.

Let everyone be clear, no-one can be certain of the scale of the fallout from leaving the EU. But there are many legitimate risks, and not even the most optimistic “leave” advocate can wave them away. We have been warned against exit – by America, China, Japan: are all these large investors in the UK to be ignored? Should we also ignore the G20? The Governor of the Bank of

England? Our military leaders? Our leading scientists and academics?

A majority of large and small businesses? Are they really all guilty of “interfering”, “scaremongering”, or being part of one enormous plot being orchestrated by Number 10? Such a notion is absurd.

Would we remain such a pre-eminent ally of the United States if we no longer had influence in the EU? Close, yes – because of trade; important, yes – because history matters; but outside the EU, part of our influence would wane. In our absence, the US would need a powerful friend within the EU – and it could no longer be us.

Our departure would not only weaken the UK but Europe, too. If the UK leaves, the EU will lose:

- the fastest growing economy in the EU;

- one of only two nuclear powers;

- a country with the longest and deepest foreign policy reach.

As a result of a UK exit, the political influence of the EU would be diminished – especially when considered against the power of the United States or China. Without the UK, Europe – the cradle of modern civilisation – would fall to a lower significance. I cannot believe that any sensible Briton wishes to divide Europe, and thus divide the West: only our enemies could gain from that, as Senator McCain has made clear in recent days. No doubt the “leave” campaign will accuse him of “scaremongering” too.

The Referendum decision on 23 June is not a prelude to further negotiation. It will be final. Our nation can either decide to be true to our history – and remain outward-looking internationalists on the world stage – or shrink to lower prominence.

It will be a fateful choice: Great Britain or Little Britain.

As our children and grandchildren look back at this pivotal moment in our history, I hope they can be proud that, in a world of uncertainties – of Daesh, of Syria, of Putin’s Russia – our country did not turn its back on Europe and cripple its authority, but chose to remain in it, reform it, and play our part in maximising British influence and European power for the common good.